The liquid-phase oxidation of an alkyl aromatic hydrocarbon to an aromatic carboxylic acid is a highly exothermic reaction commonly carried out in a vented, intimately-mixed, columnar oxidation reactor. The oxidation process comprises continuously feeding separately or in admixture an alkyl aromatic hydrocarbon, fresh and/or recycled solvent generally in aqueous solution, and catalyst components to the reactor to which a molecular oxygen-containing gas also is fed, normally at or near the bottom of the reactor. This process gas rises through the liquid contents of the reactor resulting in vigorous agitation of the reaction mixture and providing intimate contact between the alkyl aromatic hydrocarbon and the process solvent having dissolved therein the catalyst or catalyst components. The aromatic carboxylic acid produced is removed continuously through a lower exit port located at or near the base of the reactor as a slurry in the solvent which also contains soluble catalyst components. After separation of the aromatic carboxylic acid product, the solvent is returned to the reactor.
Oxygen-depleted process gas, along with minor amounts of solvent decomposition products, is removed through an upper exit port located at or near the top of the reactor. The heat of reaction is also removed through the upper exit port by vaporization of the process solvent and water generated by the reaction. The oxygen-depleted process gas and the vaporized process solvent and water comprise the reactor off-gas which is typically condensed by means of one or more condensers to separate the solvent and water for recycling to the reactor. The condensed aqueous solvent may be subjected to a water removal step prior to recycling.
The described production system can be utilized in the manufacture of aromatic carboxylic acids at excellent production rates relative to the volume of the reactor. One significant problem presented by the production system is the efficient removal of the excess water generated by the reaction since the water concentration must be held at an acceptable level, typically below 10 percent, for the reaction to continue at a reasonable rate. The reaction produces one mole of water per mole of carboxyl moiety produced. In addition, there are other side reactions which release water, i.e. the direct oxidation of the solvent to form by-products, and water may be added to the process for other reasons such as scrubbing off-gas for solvent recovery. Typically, water is removed by conventional distillation methods.
Another problem is the effective removal of the heat of reaction to control vaporization of the reactants in the reactor. A widely practiced form of heat removal is to cool the reactor off-gas in a condenser and return the cold liquid to the reactor, as disclosed in U.S. Pat. No. 4,777,287. Alternatively, the heat of reaction has been.removed by circulating a portion of the product-containing liquid at the bottom of the reactor through a heat exchanger and returning it to the reactor, as disclosed in U.S. Pat. No. 4,855,492.
To resolve the two aforementioned problems, the energy created by the heat of reaction has been utilized in the removal of excess water. In a side distillation process, the energy produced from condensation of the reactor off-gas has been used to generate steam, which is then used as part of the heat input to the side distillation column. However, this method does not effectively utilize the heat of reaction. Additional heat sources are generally required to accomplish distillation and additional heat exchangers must be added to the process.
Direct distillation of the reactor off-gas to remove water has conventionally been employed utilizing the heat of reaction. However, process limitations exist. Since the amount of distillate reflux determines the purity of the overhead distillate and the heat input to the distillation process determines the amount of reflux which the process can accommodate, the heat of reaction fixes both the amount of reflux and the purity of the overhead distillate. The heat of reaction alone is generally insufficient to obtain a desirable overhead purity which minimizes solvent loss. Therefore, direct distillation requires additional heat input.
Another effective method of water removal is by azeotropic distillation. U.S. Pat. No. 3,402,184 discloses reactor oxidation vapors being sent directly to an azeotropic distillation column in which benzene is the entraining medium. U.S. Pat. No. 4,250,330 discloses condensed solvent being sent to an isobutyl-acetate azeotropic distillation process. These methods, however, typically require expensive distillation equipment and additional heat exchange equipment. The process is more complicated and expensive since the entraining medium must be purchased, handled, recovered and replenished due to loss and degradation.
Thus, there exists a need for a method to remove the excess water generated from the reaction by effectively using the heat from the energy of oxidation without requiring additional heat input or equipment while also minimizing solvent loss.